Vehicles such as cars incorporate a structural skeleton designed to withstand all loads that the vehicle may be subjected to during its lifetime. The structural skeleton is further designed to withstand and absorb impacts, in case of e.g. collisions with other cars or obstacles.
The structural skeleton of a vehicle, e.g. a car, in this sense may include e.g. a bumper, pillars (A-pillar, B-pillar, C-pillar), side impact beams, a rocker panel, and shock absorbers. For the structural skeleton of a car, or at least for a number of its components, it has become commonplace in the automotive industry to use so-called Ultra-High Strength Steels (UHSS), which exhibit an optimized maximal strength per weight unit and advantageous formability properties. UHSS may have an ultimate tensile strength of at least 1000 MPa, preferably approximately 1500 MPa or up to 2000 MPa or more.
An example of steel used in the automotive industry is 22MnB5 steel. The composition of 22MnB5 is summarized below in weight percentages (rest is iron (Fe) and impurities):
CSiMnPSCrTiBN0.20-0.250.15-0.351.10-1.35<0.025<0.0080.15-0.300.02-0.050.002-0.004<0.009
Several 22MnB5 steels are commercially available having a similar chemical composition. However, the exact amount of each of the components of a 22MnB5 steel may vary slightly from one manufacturer to another. In other examples the 22MnB5 may contain approximately 0.23% C, 0.22% Si, and 0.16% Cr. The material may further comprise Mn, Al, Ti, B, N, Ni in different proportions.
Usibor® 1500P commercially available from Arcelor Mittal, is an example of a commercially available steel used in tailored and patchwork blanks. Tailor (welded) blanks and patchwork blanks provide a blank with varying thickness or different material properties prior to a deformation process e.g. hot stamping. The thickness variation in a tailored blank is not to be confused with (local) reinforcement. Reinforcements in this sense instead are added to a component after a deformation process.
Usibor® 1500P is supplied in ferritic-perlitic phase. It is a fine grain structure distributed in a homogenous pattern. The mechanical properties are related to this structure. After heating, a hot stamping process, and subsequent quenching, a martensite microstructure is created. As a result, ultimate tensile strength and yield strength increase noticeably.
The composition of Usibor® is summarized below in weight percentages (rest is iron (Fe) and unavoidable impurities):
CSiMnPSCrTiBN0.240.271.140.0150.0010.170.0360.0030.004
Steel of any of these compositions (22MnB5 steels in general, and Usibor® in particular) may be supplied with a coating in order to prevent corrosion and oxidation damage. This coating may be e.g. an aluminum-silicon (AlSi) coating or a coating mainly comprising zinc or a zinc alloy.
The ultimate tensile strength of Usibor® after hot stamping and subsequent quenching (i.e. with a martensite microstucture) is 1.550 MPa±150, whereas the yield strength is around 1.150 MPa±150.
In a B-pillar, an important problem is to ensure that no deformation or little deformation occurs in the middle region, as intrusion may cause damage in the vehicle occupants. One solution is having a B-pillar with zones of different thickness. Particularly, a central region (around half the height of the B-pillar) may be stronger (i.e. thicker) to avoid the aforementioned intrusion but the overall weight is thereby increased.
Another solution consists in welding reinforcements, e.g. by spot welding, to strengthen the structure. Such reinforcements are usually made of steel and even if the material is not as stiff as the material of the B-pillar, the resulting structure after joining is strengthened by the extra material. But the use of reinforcements also involves a weight increment as extra material is added to the structure.
Keeping the weight of every component of the structural skeleton under control is important, as automotive companies try to maximize weight reduction as a heavier vehicle involves not only higher manufacturing costs but also increased fuel consumption, greater difficulty when accelerating, braking and/or turning due to the high inertia of a large mass.
In order to improve the ductility and energy absorption in key areas of a component, e.g. the lower part of the B-pillar, it is known to introduce softer regions within the same component. Soft zones can improve ductility locally while maintaining the required high strength overall. Additionally, the kinematics of deformation in the case of an impact or collision may be suitably tailored by including such soft zones.
Known methods of creating regions with increased ductility (soft zones) in vehicle structural components include the provision of tools comprising a pair of complementary upper and lower die units, each of the units having separate die elements (steel blocks). The die elements are designed to work at different temperatures, in order to have different cooling rates in different zones of the part being formed during the quenching process, and thereby resulting in different material properties in the final product (soft areas). Such methods are known as in-die controlled cooling processes.
The mentioned soft zones e.g. placed in the lower part of a B-pillar, may not resist large loads and the pillar may suffer a deformation which may lead to an intrusion of the central region of the B-pillar.
In conclusion, there is a need for optimizing/improving the mechanical behaviour of a B-pillar in crash events while at the same time reducing as much as possible the weight of the same pillar.